European Patent Application No. 0 335 168 A2 describes a sensor which determines the alcoholic concentration of a fuel mixture to allow an internal combustion engine to be optimally operated. The sensor's measuring cell has an inlet and an outlet to allow the fuel mixture to flow through the measuring cell. The outer walls of the measuring cell and a central cylinder consist, at least partially, of conductive material and form a cylindrical capacitor, which is a component of an evaluation circuit. The capacitance of the cylindrical capacitor is influenced by the composition of the fuel mixture which constitutes the dielectric. Not only does the capacitance change along with the composition, but the conductance of the fuel mixture changes as well. This conductance is likewise detected by means of the capacitor's electrodes. Up until now, this arrangement had been realized in macroscopic dimensions using conventional manufacturing methods.